Qualitative und quantitative Analyse von funktionalisierten Kohlenstoffnanoröhren

Abstract

This dissertation deals on the chemical characterization of functionalized carbon nanotubes. Carbon nanotubes (CNTs) are graphene layers rolled up to tubes. Introduced functional groups are ideally exclusively covalently bound to the CNT walls. The results shown here can not strictly be generalized, but they are valid for the specific material, because CNTs are a very heterogeneous group of materials. Their physical and chemical properties as well as reactivities differ from batch to batch. In this work functional groups on CNTs are determined qualitatively and their surface concentration was quantified. Before analyzing the surface functional groups of an unknown CNT sample it has to be assured that functionalized adsorbates are not present, which could only be partially removed by washing procedures and consequently would feign a higher surface concentration of covalently bound functional groups. The surface of CNTs shows acid adsorbates called „fulvic acids“, which origin from harsh oxidation procedures and are to be taken as derivatives of polycyclic aromatic hydrocarbons because they are fragments of the CNT walls. The indirect potentiometric titration is an analytical method not only to differentiate and quantify the covalently bound acidic groups, it also measures the adsorbed acidic fragments. Values of the total acidity (G=AC+AD), the acidity of the adsorbates (AD) and the acidity of the covalently bound groups (AC) between about 100 μmol/g and 1700 μmol/g are calculated from the two equivalence points at pH≈8.1 and pH≈5.1. No experiment resulted in an optimal washing procedure leading to a permanent decrease of AD to 0 mol/g. Acidic adsorbates can be deprotonated by the NaOH, washed off and titrated in any case independent of the predecessing treatment. This shows that the resulting values from acid oxidized CNTs are least reproduceable and have to be critically considered. Furthermore it was the aim to synthesize standard or reference materials, which made the study of Lewis acid catalyzed bromination and bromoalkylation necessary. The optimal reaction parameters like duration, temperature and solvent were found. Several Lewis and Bronsted acids and reagents like bromine, α,ω-dibromoalkanes and ω-haloalcohols were tested. In the optimal procedure the CNT sample was treated with bromine in di-n-hexylether and anhydrous aluminum bromide at 200oC leading to values of between 9 at% and 23 at% bromine. The reactivity of different CNTs decreases with increasing degree of graphitization. During this bromination additional oxygen in the form of carboxyl, hydroxyl and carbonyl groups is introduced in a concentration of 6 at% up to 9 at% oxygen. This result prevents the use of high temperature brominated CNTs as starting materials for the production of standard or reference materials, although part of the introduced bromine can be substituted by nucleophiles like thiols or amines as experimentally proven. Another part of the introduced bromine is adsorbed or intercalated. The derivatization of acid oxidized carbon nanotubes has also been worked on in this thesis. They were treated with perfluorinated acylating agents of different chain length or with trifluoroethanol in the gaseous as well as in the liquid phase. It was found out, that the fluorine content of the CNT after the reaction was not proportional to the length of the perfluoroalkyl chain, but according to a sterical model the length of the perfluoroalkyl chain limits the maximum concentration βmax of functional groups, which can be derivatized. An alternative to these derivatizations with fluorine containing reagents is the here-discussed indirect uv spectrometric analysis. The functionalized CNT sample is suspended in an exactly known volume of the reaction solvent containing an exactly known amount of an uv active reagent. Its molar concentration decreases due to the heterogeneous reaction. If aliquots are removed from the reaction solution at different times and diluted, the extinction E measured at the wavelength of the absorption maximum of the reagent also decreases with the time. From the E = f(t) curve not only the surface functional group concentration can be calculated, but also the velocity constant of the reaction can be derived using a postulated kinetic model. As an example the use of uv active thiols allows the quantification of oxidizing groups and adsorbates (O2, Br2), whereas the use of an uv active styrene derivative make the differentiation between halogens and oxidizing groups possible.